1. Field of the Invention
The present invention relates to a method for processing a workpiece (an object to be processed) into an optical device or an optical device molding die by using a tool smaller than the workpiece.
2. Description of the Related Art
As a processing method for accurately finishing a shape of an optical device or an optical device molding die, there is known a shape correction processing method for processing a workpiece while causing a tool such as a polishing pad and an ion beam, which is smaller than the workpiece, to scan the workpiece. In the shape correction processing method, first, a shape of the workpiece that is still unprocessed is measured, and an amount of error of the workpiece from a target shape is grasped in advance. The amount of error corresponds to a target shape to be removed (target removal shape) for approximating the shape of the workpiece to the target shape. Further, a unit shape to be removed (unit removal shape), which is a spotting mark per unit time of the tool, is grasped in advance through preprocessing. The unit removal shape defines a removal profile of the tool with respect to the workpiece. At the same time, the unit removal shape defines a removal rate indicating a relationship between a dwell time of the tool and a depth or volume of such a removal shape formed in the workpiece.
Then, a dwell time distribution of the tool, which is set for approximating the shape of the workpiece to the target shape, is calculated based on the target removal shape and the unit removal shape. The tool is caused to scan the workpiece while controlling a scanning speed thereof according to the dwell time distribution, whereby the shape of the workpiece is approximated to the target shape (refer to Japanese Patent Application Laid-Open No. H10-337638).
The dwell time distribution is one of main factors to determine accuracy of the shape correction processing. Hence, accuracy of the removal rate for use in calculating the dwell time distribution becomes a main factor to directly influence the accuracy of the shape correction processing.
In the conventional processing method, the removal rate indicating the relationship between the dwell time of the tool and the depth or volume of the removal shape formed in the workpiece is defined only by the unit removal shape. The unit removal shape is the spotting mark per unit time, and therefore, the removal rate is defined only by the unit time. This definition is made on the premise that a depth of the spotting mark when the workpiece is processed for an arbitrary time is proportional to a processing time period of the workpiece while taking as a reference the unit removal shape that is the spotting mark per unit time. In other words, it is premised that the dwell time of the tool and the depth or volume of the removal shape formed in the workpiece are in a proportional relationship.
However, in the actual processing, the dwell time of the tool and the depth or volume of the removal shape formed in the workpiece are not always in the proportional relationship. For example, in the case where a layer like an affected layer, which is different from the workpiece in characteristics, is formed on a superficial layer of the workpiece, a removal rate at the time of removing the layer different in characteristics is different from that for the workpiece. Further, in the case where, the workpiece has characteristics that a temperature of a processed point thereof rises as the dwell time is longer, and the removal rate is changed depending on the temperature of the processed point, a removal rate at a position where the scanning speed is slow increases more than that at a position where the scanning speed is fast. As described above, in the case where the dwell time of the tool and the depth or volume of the removal shape formed in the workpiece are not in the proportional relationship, that is, in the case where the removal rate cannot be represented by a linear function, accuracy is insufficient by the above-mentioned method in which the removal rate is defined only by the unit removal shape. In particular, in processing of an optical device and, an optical device molding die, for which high shape accuracy is required, there is a problem that the shape accuracy cannot reach a target thereof owing to the insufficient accuracy described above.